Lake Wohlford Dam stable … unless big earthquake hits

ESCONDIDO —— What would it take to break Lake Wohlford Dam?
Geologists say a magnitude-7.5 earthquake from the Elsinore fault
11 miles east of the lake could cause catastrophic damage. But the
expected probability of such a dam-disrupting event is roughly once
every 2,000 years.

Two factors protect the Lake Wohlford Dam from falling apart in
a quake: its base of rock and its relatively "quiet" location,
seismically speaking, five miles northeast of downtown
Escondido.

But the top of the dam, built in 1895 and heightened in 1924, is
made of dirt. The insides of dams of similar construction can turn
to jelly in an earthquake that shakes the ground hard enough, a
process known as liquefaction.

During the 1971 San Fernando quake, which measured magnitude
6.7, the top 30 feet of the Lower San Fernando Dam slid away. Only
a thin dirt wall separated 15 million tons of water from 80,000
people living below the dam.

A few more seconds of shaking could have flooded the valley
below, according to Tom Keller, an engineer at GEI Consultants in
Carlsbad who headed a 1987 study of the episode funded by the Army
Corps of Engineers.

"It was a lucky aversion of disaster," he said.

If the Lake Wohlford Dam were to fail, most of central
Escondido, including City Hall, would be flooded within three
hours, according to city emergency plans.

That's why city staff say they check the structural integrity of
the dam every week, and state inspectors examine it twice a year.
They have monitored the pressure and water content at a score of
test locations inside the dam for decades.

The city's most recent assessment of the dam's strength found in
2004 that it could withstand the "maximum credible earthquake" that
would come from the Elsinore fault, the closest to Escondido.
However, the power of that "maximum" has been the matter of some
debate, and there's always room for extra reassurance.

For example, state officials this summer ordered the level of
Lake Perris in Riverside County to be lowered after they determined
that the reservoir's dam, made of earth, would be unstable in the
event of a major quake coming from the San Jacinto fault five miles
to the east.

Escondido city officials are preparing to do an additional
"liquefaction" study next year. They have asked Keller and GEI
Consultants to take a look at the possible response of the material
in Lake Wohlford Dam to extreme stress.

"There's no cause for alarm," Keller said. "It's time for a
check-up."

The study could cost as much as $150,000 and take a year if it
is approved by the City Council later this month, said Glen
Peterson, the city's assistant utilities manager.

Built with rock, topped with dirt

Originally called the Bear Valley Dam, the structure ensured a
water supply for the young agricultural community of Escondido. It
was built under the supervision of contractor I.E. Doty of
Nebraska, who had experience building railroads and irrigation
ditches across the western United States.

The then-76-foot-high dam was formed by granite blasted from the
hillsides during the construction of the flume, which carried water
from the San Luis Rey River 14 miles away. One of the roughly 75
men who worked to build the dam died in a premature explosives
discharge, according to Escondido historian Frances Beven Ryan.

The total cost at the time was about $450,000, including the
construction of the flume.

Redwood planks were used to line the lake-facing side of the
dam, with tar sealing the wood like on a ship's hull. The dam was
strong enough to survive torrential rains in January 1916, which
sent water from Bear Valley Lake flowing over the dam.

In 1922, the Escondido Mutual Water Co. decided to raise the dam
to hold additional water from recently created Lake Henshaw,
doubling the lake's capacity. Engineer Harry Hawgood recommended
the then-common technique called "hydraulic fill": essentially,
packing mud behind the original dam.

Photographs of the construction from the Escondido Historical
Society show a pipe conveying a slurry from the lake bed into a
pond between the original dam and a mound behind it. As the solids
settled, water was pumped out.

The now-100-foot-high, 420-foot-wide dam, renamed after
long-serving water company board member Alvin Wohlford, was
completed in 1924 with a maintenance road across the top. The dam
has a volume about two-thirds of the Transamerica building in San
Francisco.

What to expect from Elsinore

Engineers don't construct hydraulic fill dams anymore because
they're not as strong as such modern materials as rolled
concrete.

Still, city plans say the Lake Wohlford Dam is strong enough to
withstand "peak ground acceleration" of up to 0.27 g, or just over
a quarter the strength of gravity.

That's equivalent to shaking from a magnitude-7.3 quake from the
Elsinore fault. In recorded history, the Elsinore fault, which runs
through Julian and close to Lake Henshaw, has never produced a
quake of that magnitude, according to the Southern California
Earthquake Center.

The last major rupture was in 1910, generating a magnitude-6
quake. An extension of the fault in Mexico did produce a
magnitude-7 quake in 1892.

"The Elsinore fault is not inactive," said Tom Rockwell, a
geology professor at San Diego State.

However, a large earthquake —— magnitude 7.5, for example —— is
probable roughly every 2,000 years on the Elsinore fault, he
said.

That compares to every 100 to 200 years on the more distant, but
more active San Andreas or San Jacinto faults, Rockwell said.

Escondido officials would have had to replace or reinforce the
Lake Wohlford Dam more than 20 years ago if the Elsinore fault were
more seismically active.

In 1982, the state Division of Safety of Dams told the city of
Escondido that it had to prepare for a magnitude-7.5 quake. That
would have required the city to spend millions to strengthen the
dam or keep the reservoir level lower.

State officials backed off after Nevada earthquake expert D.B.
Slemmons advised them that a 7.5-sized quake from the Elsinore
fault was only possible if most of the fault ruptured at the same
time. This was considered unlikely because of the fault's twists
and turns.

Monitored carefully

Within Lake Wohlford Dam, 23 observation wells measure the water
level inside. City staff check the wells five times a week when the
lake's water level is high, Peterson said. The wells provide an
estimate of the pressure within the dam, substituting for pressure
meters built into dams constructed later, such as the dam holding
Lake Dixon.

"They're less sophisticated, but they still serve the same
purpose," he said.

In addition, city workers every week monitor water seepage
through the dam, and test the alarm that would sound if the
downstream level of Escondido Creek rose.

The requirement for next year's liquefaction study is coming
from the Federal Energy Regulatory Commission, which regulates
hydroelectric dams.

In the study, engineers will bore into the dam to determine how
dense the material is and how much clay, sand or silt it contains,
Keller said. The study will also include physical tests of the dam
material and a review of the latest seismic data, including faults
farther away than Elsinore, such as the San Jacinto, he said.

Although city officials openly discussed the strength of the dam
and emergency evacuation procedures, they said the regular report
on its structural integrity is not available to the public because
of security concerns. A fence and chain keep casual visitors
out.

Peterson noted that determined people could probably damage the
dam, even if the Elsinore fault couldn't.

To be sure, somebody already tried. In April 1917, just weeks
after the United States entered World War I, a mysterious explosion
ripped out the dam's spillway planks. Divers repaired the resulting
leaks, according to a 1965 newspaper account.